1. Field of Invention
This invention relates to a self-aligned silicide process, and more particularly to a process for forming a self-aligned metal silicide layer such that the metal silicide layer is formed smoothly despite the use of a high processing temperature. Also, the profile of the metal silicide layer can be kept in a stable condition even when the silicide layer is subjected to subsequent high temperature processing operations.
2. Description of Related Art
In the design of integrated circuits, as the level of integration for semiconductor components is increased, resistance in the source/drain terminals of a MOS component will correspondingly increase. When the resistance is increased to a level comparable to the resistance of a MOS channel, in order to reduce the sheet resistance in the source/drain terminals as well as to maintain integrity for the shallow junction between the metal layer and the MOS component, a process known as the self-aligned metal silicide process is often applied in the fabrication of very large scale integration (VLSI) circuits for line widths smaller than about 0.5 .mu.m.
FIGS. 1A through 1D are cross-sectional views showing the progression of manufacturing steps for a conventional self-aligned silicide process. First, referring to FIG. 1A, a silicon substrate 100 with the main parts of an integrated circuit already formed above, for example, a field oxide layer 110, a gate 120 and source/drain regions 130, is provided.
Referring next to FIG. 1B and FIG. 1C, in the subsequent steps a titanium (Ti) layer 140 is formed on the field oxide layer 110, the gate 120 and the source/drain regions 130. Thereafter, a thermal processing operation, for example, a rapid thermal processing, is performed so that part of the titanium layer 140 reacts with the polysilicon on the upper surface 121 of the gate 120 and the silicon layer on the upper surface 131 of the source/drain terminals 130 to form titanium silicide (TiSi.sub.2) layers 150.
Referring next to FIG. 1D, a wet etching method is used to remove the reacted or residual titanium layer 140 (the residual titanium layer may not necessarily be in the same original form), and leaving behind a layer of titanium silicide 150 on the gate 120 and source/drain regions 130.
A self-aligned silicide process not only can establish a low resistance metal silicide layer, for example, a titanium silicide layer, on silicon and polysilicon surfaces, but also can do so without photo lithographic processing operations. Therefore, it is a very attractive contact metallization procedure. However, owing to the high temperature needed in the process, the siliciding steps are hard to control. Although rapid thermal processing is frequently used in such self-aligned silicide processes, process yield is somewhat low due to the restrictions imposed by the level of maturity in technical applications and other unresolved manufacturing problems.
FIG. 2 shows a cross-sectional view of the titanium silicide layer during subsequent high temperature processing operations after its formation by a conventional self-aligned silicide process. Referring to FIG. 2, after the formation of a self-aligned silicide layer by a conventional process as shown in FIG. 1D, a dielectric layer 260 and a passivation silicon nitride (Si.sub.3 N.sub.4) layer 270 are sequentially formed on the field oxide layer 110, the gate 120 and the source/drain regions 130 above the silicon substrate 100. After that, high temperature processing operations are performed to convert the original titanium silicide layers 150 into titanium silicide layers 250a, 250b and 250c, respectively. For example, if the component is a logic device, then high temperature processing operations are necessary for the formation of its peripheral memory components. These subsequent high temperature processing operations damage the good titanium silicide layer 150 and transform it into somewhat irregular forms 250a, 250b and 250c. As a result, the production yield is greatly reduced by using the conventional self-aligned silicide process.
In light of the foregoing, there is a need in the art for an improved process.